This invention relates generally to a multichannel record disc reproducing system and apparatus, and more particularly, to a system and apparatus for reproducing multichannel record discs, while restraining or suppressing noise components in the demodulation.
A discrete four-channel record disc system was previously proposed by one of the present applicants, Nobuaki Takahashi and was patented in the United States as U.S. Pat. No. 3,686,471. Here, a direct wave is formed from the sum signal of a pair of two channels. An angle-modulated wave is obtained by angle modulating a 30 KHz carrier wave, responsive to a difference signal derived from a pair of two channels. The direct and carrier waves are multiplexed and recorded on the respective side walls of the disc sound groove.
In picking up and reproducing a recorded signal from this multichannel record, there is a need for taking out the angle-modulated difference signal from the picked up signal. It is separated from the direct wave sum signal, in order to demodulate the angle-modulated difference signal. It is to be understood that the direct wave sum signal has a frequency band ranging from 0 to 15 KHz, while the angle-modulated difference signal has a frequency band ranging from 20 KHz to 45 KHz.
In general, abnormal noises, generated at the time of the reproducing of a multichannel record disc, can be classified broadly into the following two kinds, depending on the cause.
1. Abnormal noise is caused by wear of the sound groove of the disc. The carrier level of the angle-modulated wave drops greatly. The noise level becomes higher than the carrier level.
2. Abnormal noise is caused by the nonlinearity of the mechanical systems of the cutter in the recording system, the pickup in the reproducing system, and sound groove of the record disc when the level of the direct wave sum signal is extremely high, and particularly when the level of the high-frequency component is high.
One of the present applicants, Nobuaki Takahashi, has previously proposed various systems for preventing the reproduction and generation of the above enumerated noises, in demodulated signals. For example, if an angle-modulated wave is partially lacking, there is an equivalent of a deviation of the angle-modulated wave to a low frequency. For this reason, in one proposed system, this frequency deviation is detected, and muting is applied responsive thereto in order to shut off the signal. In another system, the above mentioned deviation toward the low frequency is detected. In response to the resulting detection output, the loop gain of a phase-locked is decreased thereby to constrict the lock range and thereby to prevent the phase-locked loop from locking to the noise component.
In accordance with these prior proposed systems, impulse noises, arising from causes such as scratches and dust in the disc sound groove, are effectively suppressed with no great problems. However, if the level of a sum signal is high, particularly in a high frequency band, the angle-modulated wave signal is continuously disturbed. The application of one of the above mentioned proposed systems continuously gives rise to an attenuation or a cutting off of the difference signal component. High fidelity reproducing cannot then be carried out. Furthermore, these proposed systems are also accompanied by other problems, such as unsatisfactory sound source localization.